Concentration of phosphatic material



Dec. .10, 1957 C. A. HOLLINGSWORTH CONCENTRATION OF PHOSPHATIC MATERIALFiled 001:. 15, 1954 Conc. Bin- FIG. I

Wusher Undersnze Waste l6 Froth roth Frofluv Tails to Wade 0 23%] qINVENTOR CLINTON A. HOLLINGSWORTH B @mamlfinnmgmpx ld ATTORNEYSCONCENTRATION OF PHOSPHATIC MATERIAL Clinton Holiingsworth, Lakeland,Fla, assignor to Smtth DougiaSs-Company, Incorporated, Nortolk, Va., 21corporationoftvirginia' Application Qctober 15, 1954,.S1erialNo. 462,470

8 Claims. cr. 209mm- This invention relates to the froth. flotationproce s for concentrating .phosphatic material and more particularly tothe separation of silica from phosphaticmaterial by froth flotation, andhas for its object. the provision of an improved methodof frothflotation .for separating silica from phosphatic. material.

The main .phosphatic. constituent. of Florida Phosphate res Patent rock,as well-as most native. phosphate .mineralscontaining fluorine,is-fluorapatite (usually referred to as apatite) commonly believed tobeacombination of trlcalc um phosphate and calcium fluoride (.Ca F (PQ Thegrade of the rock .is determined by its tricalciumphos- .phate content,which isusually. designated in the industry as bone phosphate of lime(BPL). Silicais one ofthe chief valueless or gangneconstitnentsotphosphate rock and in the phosphate industry. is commonly determined andreported as insolub1es.(insol.-). For-manypurposes,

it is presently the common. practice throughout the Florida phosphatefield to double-floatthe undersi-ze fromnthe washing plant. That is,after concentration ott-heraw rock bywashing andthelike, the undersizeorfinesfrom the washer is further concentrated in two stages-of frothflotation. In the first stage,.the .deslimedflo-tationteed isconditioned with caustic soda, fuel oiland a iattyacid such as tall oil,and the conditioned feed is su'bjectedto froth flotation where ,aphosphate product (commonly called a rougher concentrate) is floated andthe linderflow (largely silica) is discarded to waste. Thezrougherconcentrate normally contains v8 to of silica, and hence its phosphategrade (BPLlis too low to be of much practical value. Accordingly, .therougher concentrate is de-oiled by scrubbing with sulphuric .acid

followed by desliming. The de-oiled'product (commonly called the aminecell feed) is subjected to .the second stage of froth flotation in thepresence of a strong cationic collecting agent, such as an amine, wheresilica is floated and discarded to waste. The underflow of the secondstage of flotation is a high grade phosphate product of low silicacontent, and is transferred'to storage. In the phosphate field, thephosphate product. is called the concentrate regardless of whether it isthe floated product or the underflow, and hence the silica float iscalled tailings or tails.

Phosphate ores are friable and tend toproducedeleterious amounts ofslime during flotation treatment, especially in machines of theagitation type. .Slime has been found to be particularly objectionablein the amine circuit flotation, since the presence of slime necessitatesan increased amount of the relatively costly amine reagent with anattendant loss of phosphatic material in the silicatailings in order toobtain the desired low silica content in the phosphate concentrate. Intheaforementioned twostage practice, considerable slimes are produced inthe .the pulp column, through a porous medium'at the bottom Icescrubbing of the rougher phosphate concentrate with sulphuric acid,andjhence the de-oiled product is deslimed preparatory to the secondstage of froth flotation. But even then, the types, of flotationapparatus presently used in the Florida phosphate field produceobjectionable amounts of slime during the flotation treatment due to theviolent agitating. action. of the impeller.

In my copending application for Letters Patent of the United States.Ser. No. 452,177, filed AugustfZS, 1954 (Patent ,No..2,.7;5'.8,7,14 .Ihave described a methodof pneumatic-hydraulictflotation peculiarlyadapted for phosphate minerals because of its highly effective aerationwithout. creatingcslime in. objectionable amount. I have .fiou-n'dthatwhenflotation 'is carried out in accordancewith-this method, thefirststageiof the present common flotation practice .(in-whichphosphateis floated.) can be replaced ,by arougher froth flotation step in whichsilica is floated, .therebyeliminating .the present tie-oiling treatmentof the tougher- ,phosphate concentrate. The

.presentinvention makesuse of this discovery, and in its broad .aspectinvolves arr-(improved method of separating silica, as,a.fl.oat,-.from-phosphaticmaterial by froth .in-the presence of .ast-rong cationic collecting agentand silicais floated'in each. Each step.of flot ation iscarr'ied ou i fla rela ve y e p olunin -.of pu p. angthedow iward velocity of pulp flow in the columniis. substantiallydecreased and thecreationof slimesisth'ereby minimized byjintroducingwater .ina fine state of .subd'itiisionfmto thereof; in accordance withthe invcntionof ,my aforementioned ,appliQafiQIL. .In addition .to,providingitwo separateand similar steps of froth. flotation,-in whichdifferent cationicjlotationagentsmay housed if desired, the method of tthe invention contemplates particlegsifzing of the rougher phosphateconcentrateintoja coa'rseieed and fine feedfor the second stepot frothflotation, thereby permitting, if desired or necessary reroptimumresults, the use of different cationic-flotation agentswwith the eeds ofdifferent particlesizc.

The foregoing andothernoyel.features .of theinvention Wi b betterunderstood from the followingdescription t aken'in conjunction'with theaccompanyingtdrawn in which:

Fig. 1.,i s a .diagrammaticflow sheet embodying the principles ,otthe,invention, and 1 Fig. .2,,.i s".a. side. elevation,..mostly in: section,of the pneumatic-hydraulic flotation cellor apparatusdescribed in myaforementioned application and especially adapted for practicingthepresent invention.

Referring-to thefflows'heetot Fig. 1,. the feedtothe flotation plantisthe undersizeor fines from the phosphatelwasherplant having aparticle:size'norrnallyminus 14.m.esh, i. e. substantiallyall througha.14 mesh standard Tyler screen. The following is..a typical screenanalysis, of the flotation. plantfeed orheads:

Thus, .theflotation heads priced is predominatelyfi. .e.

.morethanhalf) .minus .20rmeshand on- 65 mesh. The

(BPL). The washer undersize is first dewatered in a large hydroseparator5, the overflow of which is discharged to waste, and the underflow pulpis pumped to storage bins 6. From the storage bins, the pulp is pumped,as required, to a surge bin 7, the overflow of which goes to arelatively small hydroseparator 8. The underflow of the surge bin goesto a rake classifier 9, the overflow of which goes to the hydroseparator8. The rake product is delivered to a deslimer 10, the overflow of whichis returned to the large hydroseparator 5. The settled underflow of thehydroseparator 8 is pumped to the deslimer 10.

The deslimer pulp is delivered to two series (11 and 12) of rougherflotation cells, arranged in tandem, that is the phosphate underflow ofthe cells 11 is the feed of the cells 12. The silica froths of bothcells 11 and 12 are combined and fed to froth-cleaning flotation cells13. The silica froth (tails) of the cells 13 is discharged to waste, andthe underflow of these cells is delivered to a catchall sump ormiddlings tank 14.

The underflow of the cells 12 (rougher phosphate concentrate) isdelivered to a storage sump, from which it is pumped, as required, toparticle sizers in which products of two different particle sizes areproduced, namely coarse and fine, as feed for the flotation cleanercells. The following are typical screen analysis of these sizedproducts:

+20 mesh 20 +35 -35 +65 65 +150 150 Coarse teed, percan Fine feed,percent.

Thus, the coarse feed is predominately (i. e. more than half) minus 14mesh and on 35 mesh, while the fine feed is substantially all minus 35mesh and predominately minus 65 mesh.

The coarse and fine feeds are delivered to separate rinsers 15 and 16,respectively, in the nature of deslimers. The underflow of the coarsefeed rinser 15 is delivered to two series (17 and 18) of flotation cells(cleaners) arranged in tandem. The underflow of the fine feed rinser 16is delivered to two series (19 and 20) of flotation cells (cleaners)arranged in tandem. The underflows of the cells 18 and 20 (final cleanerconcentrate) are combined and deliverd to a dewatering screw classifier21, and the dewatered phosphate concentrate is discharged to storagebins. The screw classifier overflow is delivered to the middlings tank14.

The silica froths of the cells 17 and 18 are combined and retreated in afroth cleaning cell 22, while the silica froths of the cells 19 and 20are combined and retreated in a froth cleaning cell 23. The silicafroths (tails) of the cells 22 and 23 are discharged to waste, and theunderflow of these cells is conveyed to the middlings tank 14. Middlingsfrom the tank 14 are pumped to the storage bins 6 for the dewateredwasher undersize at the head of the flotation plant.

As a general indication of the distribution of the flotation cells inthe plant, where there are 6 rougher cells in each series 11 and 12, 4froth-cleaning cells 13 (of approximately the same size) are provided.In each of the series of cleaning cells (1718 and 19-20) there are 2cells, and 1 froth cleaning cell (22 and 23) is provided for each seriesof cleaning cells.

The pneumatic hydraulic flotation apparatus or cell illustrated in Fig.2 of the drawings is built in separate units or sections which arevertically assembled and secured together to form the completeapparatus. The apparatus illustrated in the drawings is built up of atop unit A, an intermediate unit B and a bottom unit C. The top andbottom units may advantageously be about 2 feet in depth and the depthof the intermediate unit (or units) may then advantageously be from 2 to3 feet. As many intermediate units B may be assembled in the completeapparatus as required to give the desired over-all depth of pulp column.Usually the assembled cell will accommodate a pulp column of at leastsix feet in depth. Each unit is provided with at least one group ofhorizontally disposed air-diffusers 25, so that when the units areassembled the various groups of air-diffusers are superposed andvertically spaced. The air-diffusers are of such construction that airis uniformly introduced into the pulp column in a very finelydisseminated state over the entire cross-sectional area of the pulpcolumn.

Each unit comprises a metal frame 26 to which are bolted or otherwiseappropriately secured side and end members 27 and 28, respectively. Theside and end members may advantageously be transparent thermoplasticresin plates, such as Rohm and Haas Companys Plexiglas, through whichthe action of the air-diffusers and the condition of the pulp column canreadily be observed. Where the side and end members are of metal orother opaque material, the side members are preferably provided withtransparent windows at levels opposite the air-diffusers, as indicatedby the dotted lines 27'. The juxtaposed ends of the frames 26 ofadjacent units have peripheral flanges 29 for bolting or otherwiseappropriately securing the units together.

The bottom unit C has a hydraulic compartment 30' at its lower end whichcommunicates with the main upper portion of the unit through a porousmedium 31. While the porous medium is shown in the drawings asconsisting of a punched metal plate covered with a bed of lead shotabout one inch in depth, the medium may consist of porous tile, punchedmetal plate, metal screen, heavy canvas and the like. The hydrauliccompartment 30 is connected to a suitable source of water through a pipe32 having a suitable control valve (not shown).

One end wall 23 of the bottom unit C has a lower pulp discharge opening33 and an upper pulp discharge opening 34 communicating with a commonpulp discharge chute 35. The discharge opening 33 is just above theporous medium 31, while the discharge opening 34 is near the top of theunit, and the horizontally disposed air-diffusers are about midwaybetween the two openings with one diffuser positioned in the dischargechute 35. The chute 35 extends across the width of the end wall 28, andis provided at its bottom with a pulp discharge pipe 36. The rate atwhich water is introduced into the hydraulic compartment 30 is soproportioned with respect to the openings 33 and 34 and the pipe 36 asto be substantially the same as the rate at which water is discharged inthe mineral pulp through the pipe 36.

The top unit A has a feed hopper 37 at the top of one end wall and anadjustable froth overflow weir 38 at the top of the outwardly flaringopposite end wall 28. A perforated plate 39 is horizontally positioned ashort distance (normally about 2 inches) below the level of the Weir 38,and extends from the inside bottom of the feed hopper 37 to the first ofthree vertically disposed (i. e. depending) and horizontally spacedtransverse baffles 40. The outward flare of the end wall 23 is such thatthe top of the unit A is about twice the length of the bottom of theunit, and the horizontally spaced baflles 40 are positioned above theflared end wall. A transverse perforated baflle 4-1 depends from aboutthe center of the horizontal perforated plate 39, with one air-diffuser(nearest the discharge weir 38) on one side of the baflle 41 and theother air-diffusers of unit A on the outer, or feed hopper, side of thebaffle 41.

The intermediate unit B is provided with a vertical chamber or channel42 along that end wall adjacent the flared end wall 28' of the top unit.The channel 42 communicates at its top with the top unit A and at itsbottom with a similar but shorter channel 43 at the upper end of thebottom unit C. The channel 43 communicates through an opening 44 withthe unit C at a level slightly above that of the group of air-dilfusersof the unit, and the channel 42 communicates through an opening 45 withthe ass-sees unit B at a level slightly above that of the group ofairdiifusers in the"unit The channels 42-and" 43*areof the same width astheir respective units, and are provided with transverse dampers 46 and47, respectively, forregulating the pulp circulation.- The dampers areadapted to be adjusted by a removable-wrench; shownin dotted lines.

Each of theflotation cellsin the flowsheet of Fig. l is of the typeshown in Fig-2 In each the feed-of phosphatic material, inthe fer-m ofan a'que'o'us mineralpulp, is delivered through the hopper- 37 into arelatively deep column or body of pulp undergoing froth flotationtreatment. The flotation reagent (orre'a gentsD may be mixed withthepulp before or as it is-introdu'ced into the'hoppcr, or part or allof the reagent maybe-introduced at'one or or more lower levels inthe:pulp c'olumn.- Where the pulp has been well deslimed, itispreferable to add the reagent with the feed but whenth'efeedc'ontaihsany slime, some or all of the reagent shouldbe introduc'ed 'atone ormore lower levels in. the pulp columnc Aeration takes place at thethree (or more) differentlevels atwhich the groups of air-diffusers arepositioned, and is extensive in sectional area and gentle anduniform:..ini character. Each group of air-diffusers discharges adenseem'ass oflextremely fine, uniform. and gentle streams of air intothe:- pulp column over the: entire cross-sectional'area thereof.

Bubbles from the rising air streams attach themselves to thesilicaparticles: selectively conditioned. by the cationic. flotationagent to float, and carry such particles into a layer or column of frothoverlying the surface of the pulp body. The perforatedplate39= is. atapproximately the liquid level of the pulp column,. and serves'tominimize surface turbulence so that. the overlying layer of froth isrelatively quiescent. Asthe'froth passes longitudinally beyond theactive influence of the top air-diffusers toward the overflow weir 3'3,it moves over a' relative quiescent body of pulp (adjacent andintdirect'communication with the pulp column") where opportunity isafforded phosphate particles to drop out-of. the froth layer. Thus ineffect there areinthe'top unit A an active aerated zone and an adjacentquiescent settling zone. The single air-diffuser in the latter zone'provides aeration; for returning to the froth layer any silica particlesin the-immediate vicinity of the boundary between the two zones. Thedepending transverse battles 40-and 41 diver3t descending phosphateparticles away from'theactive aerated-zone of the pulp column andgenerally inxthe direction of tl e open top of the channel 42. Thebattle 41: extends more deeply into the pulp column than do-the baflles'4h, because of its proximity to the air-diffusers. The depending baffles40 and 41 further tend to minimize circulation of the pulp body directlybelow the froth layer, and thus induce a quiescence in the pulp body'that is beneficial in guiding the phosphate particles (dropped from thefroth layer) toward the open end of the channel 42..

There is a natural downwardcirculation of pulp in the channels 42 and43, due to the upward impetus that is given the pulp column in the unitsB and C by the rising air streams from the difiusers 25 in those units.By ad-- justment of the dampers 46 and 47, any desired portion. of thedownwardly circulating pulp can be diverted from the channels 42 and 43through the openings 45' and 44 into the units B and C, respectively.Thus, the channels provide means for short-circuiting phosphateparticles to a lower level of the pulp column, and for returning anysilica particles to the pulp column, at levels of active aeration. Inthis manner, as well as in the characteristic arrangement of the airdifl'users at several different levels in a relatively deep pulp column,silica particles are given several opportunities of attaching themselvesto rising air bubbles thereby minimizing the amount of such particlesincluded in the underflow.

The introduction of a controlled amount of Water from the hydrauliccompartment 30 into the lower portion of the pulp column is acharacteristic feature of the method,

T he water -so introduced into the pul'p column iscon veniently calledthe hydraulic water. It promotes=circulanon within thepulpcolumnandprolongs the period of suspension of silicaparticles-andgives them better, longer and more frequent opportunity forattachment torising air bubbles; The hydraulicwater alsoprevents sandingtlp ofthecell; since the pulp density of the under-flow-dischar-gods lowered bythewater entering the pulp-column in the region of the und'erflowdischarge. The pulp density of the feedto-the' sells 1 1,174 and-l9-i's' generally within the range of- 30 to-7 5.% solids andishigherthan the pulp density of-thei-r underflow discharge, which is generallywithin the range of20 to-40% solids. Generally speaking, the averagepulp densitywithin the cells is roughly between 15 and 35% solids. From25 to 50% of thewater of thepulp column inthese cells is introduced withthe feed, and the remaining percent (that is from to 50% is introducedas hydraulic water. The volume of hydraulic water introduced into theunit C (per unit oftime) is substantially equivalent tothe volume ofwater in the underflowi discharge from-the pipe 36 in thesame timeperiod;

As phosphate particles descend into the unit C, some will be-carried bythe pulp stream through the upper openin'g 3'4. into thedischarge chute35. The simple airdiffuser near the top of the chute offers a furtherand last: opportunity for any silica-particles to be carried upward byattachment to rising air bubbles. Similarly, ,the remainderofthelowerrnostair-diffusers in the unit C promote-final cleaning of thepulp stream of silica particlesbefore' discharge of the: pulpthrough thebottom opening 33 intothe ch'ute-35c Withdrawing with: the; underflowdischarge of substantially the same volume: of water as introduced intothe unit C through it's porous bottom decreases the downward 'velocityof pulp flow and thus provides and insures a relatively quiescent columnof pulp above the unit C. This relatively quiescent column of" pulpprovides better and: longer contact of silica particles with. airparticles, and thusip'romotesseparation of silica. and phosphateparticles andiminimizes slime production. In this quiescent column ofpulp there are only-gentle circulatory motions resulting from theeffects of the rising air streams and the chaunel'stll and 43, and hencelittle, if any, tendency to create slime.

Normally, the froth column or bed will be from 1 to 5 inches in depth.The depth of froth depends to a large extent upon. the type of flotationreagents employed, some yielding: a voluminous froth while others givevery little froth. The depth of froth can be controlled to some extentby the use of different frothingagents, surfaceactive agents,depressants etc. The perforated frothsubduing plate 39 is normally about2. inches below the level of the adjustable weir 33. The pulp (that isliquid) level may be raised or lowered, in relation to the perforatedplate 29, by adjusting the number of filler-bars that make up theadjustable weir 28. Thus, the pulp level should be lowered for a shallowfroth bed and raised for a deep froth bed.

It is presently the common practice in the Florida phosphate field tosize the raw flotation feed (washer fines) in order toprovide (in thesecond stage of flotation) a sized amine cell feed, since largerparticles of silica do not float with the averagequantity of reagentsused, and an increase in the quantity of reagent required to float suchlarger silica particles results in floating large amounts of phosphatewhich are lost in the silica tails. In accordance with the presentinvention, the feed to the first bank of rougher flotation cells 11 isunsized washer fines, and a large proportion of the silica gangueis'removed in this first step of flotation and before the rougherphosphate concentrate is sized preparatory to the second, step offlotation. By dividing the rougher phosphate concentrate intoproductsoftwo different sizes, different amounts of reagent, or differentreagents,can be used with the 7 coarse feed and the fine feed to give the optimumresult with each.

I have found that all silica does not appear to respond in the same Wayto flotation treatment. For example, one type of reagent floats part ofthe silica best while another type of reagent floats the remaining partof the silica best. By floating in two steps of silica flotation,advantage can be taken of this phenomenon. Thus, a different type ofcationic collector may be used in the second or cleaner step of theinvention than used in the first or rougher step, or perhaps diflerentamounts of the same cationic collector in the two steps of flotationWill give the optimum results. In the present preferred practice of theinvention, the first step of flotation of unsized feed (in cells 11, 12and 13) is carried out with the reaction product of sulphuric acid and amixture of a fatty amine and liquid hydrocarbon, such as described andclaimed in the copending patent application of Jordan L. Wester andmyself, Ser. No. 443,692, filed July 15, 1954. Typical of such amodified amine reagent is the reaction product of 4.76% (by weight) ofsulphuric acid and a mixture of 19.05% of Armour and Co.s Armoflote S(consisting of about 70% free fatty amine and about 30% nitrile) and76.19% of crude turpentine. The second step of flotation of sized feedis then carried out with an unmodified amine (i. e. one not reacted uponby sulphuric acid), such as Armoflote S or other suitable fatty amineflotation reagent or other type of cationic collecting flotation agent,usually in conjunction with a suitable frothing agent such as pine oil,crude turpentine or one of the so-called alcohol frothers (e. g.di-isobutyl carbinol). Different quantities of the same unmodifiedamine, or different cationic collectors, may be used with the sizedcoarse feed and fine feed, respectively, to give the optimum result witheach. While I noW prefer cat-ionic collectors of the amine type, othertypes of cationic collectors are now commercially available for floatingsilica and may be used in practicing the invention.

The following table illustrates the broad ranges of BPL and insol. inthe principal products of the method, and also gives the more typicalBPL and insol. contents generally obtained in plant practice.

Product Approx. Approx.

BPL lnsol.

Percent Percent Initial flotation feed 20-50 73- Generally (around) 3555 Rougher phosphate con 55-71 30-8 enerally (around) 65 16 Silica floatof cells 13 2-10 96-87 Generally (around)... 6 92 Coarse feed to c ls 1755-71 30-8 Generally (around) 65 15 Fine feed to cells 19....... 55-7130-8 Generally (around)... 65 15 Coarse phosphate conc. (cell 18). 74-786-3 Generally (around) 76 4 Fine phosphate cone. (cell 20) 74-80 6-2Generally (around) 77 3 Coarse tails from cell 22 -20 93-73 G enerally(around) 88 Fine tails from cell 25... 5-20 93-73 Generally (around) 1088 In the first rougher step of the invention, the BPL recovery is inexcess of 90%, and usually 95% and higher. In the second cleaner step,the BPL recovery is usually at least 95% and frequently as high as98-99%. The overall BPL recovery of the complete method, that is theamount of phosphate in the initial unsized washer undersize that isrecovered in the final cleaner concentrate, is usually at least 85%, andfrequently 90 to 95%. The slime loss in the method of the invention isfor all practical purposes negligible, contrasted with a slime loss ofabout 10% in the heretofore common double float practice of the Floridaphosphate field. The lost slimes have a relatively high BPL content, andconsequently the overall BPL recovery of the flotation plant in thedouble float practice seldom exceeds 80%. The slimes are delivered 8 tothe pond, along with the silica tails, and no attempt is made to recoverthe phosphate in the bed of settled solids.

I claim:

1. The method of separating silica from an unsized silica-containingphosphate product which comprises subjecting the deslimed and unsizedphosphate product to a first rougher step of pneumatic froth flotationin the presence of a cationic collecting flotation agent and in thecourse of which a silica froth and a rougher phosphate concentrate areobtained, subjecting said rougher phosphate concentrate to a sizingtreatment to obtain a product of relatively coarse particle size and aproduct of relatively fine particle size, and subjecting each of saidsized products to separate cleaner steps of pneumatic froth flotation inthe presence of a cationic collecting flotation agent, each cleaner stepof pneumatic froth flotation being conducted in the presence of adifferent cationic collecting flotation agent than that present in thepreceding rougher step of pneumatic froth flotation.

2. The method of claim 1 in which the rougher step of pneumatic frothflotation is conducted in the presence of the reaction product ofsulphuric acid and a mixture of a fatty amine with a liquid hydrocarbonand the cleaner step of pneumatic froth flotation is conducted in thepresence of a fatty amine flotation agent unreacted upon by sulphuricacid.

3. The method according to claim 1 in which one of said sized productsis subjected to a cleaner step of pneumatic froth flotation in thepresence of the same cationic collecting flotation agent as that presentin the preceding rougher step of pneumatic froth flotation, and in whichthe other of said sized products is subjected to a cleaner step ofpneumatic froth flotation conducted in the presence of a differentcationic collecting flotation agent than that present in the precedingrougher step of pneumatic froth flotation.

4. The method of separating silica from an unsized phosphate productcontaining at least two types of silica which comprises subjecting thedeslimed and unsized phosphate product to a first rougher step ofpneumatic froth flotation in the presence of a cationic collectingflotation agent and in the course of which a silica froth and a rougherphosphate concentrate are obtained, subjecting said rougher concentrateto a sizing treatment to obtain a product of relatively coarse particlesize and a product of relatively fine particle size, and subjecting eachof said sized products to separate cleaner steps of pneumatic frothflotation in the presence of a cationic collecting agent formulated tocollect the type of silica predominating in each of said sized products.

5. The method of separating silica from an unsized phosphate productcontaining at least two types of silica which comprises subjecting thedeslimed and unsized phosphate product to a first rougher step ofpneumatic froth flotation in the presence of a cationic collectingflotation agent and in the course of which a silica froth and a rougherphosphate concentrate are obtained, subjecting said rougher concentrateto a sizing treatment to obtain a product of relatively coarse particlesize predominately minus 14 mesh and on 35 mesh and a product ofrelatively fine particle size substantially all minus 35 mesh andpredominately minus 65 mesh, and subjecting each of said sized productsto separate cleaner steps of pneumatic froth flotation in the presenceof a cationic collecting agent formulated to collect the type of silicapredominating in each of said sized products.

6. The method of separating silica from a mixture of phosphatic materialand silica which comprises subjecting an aqueous pulp of the deslimedand unsized mixture to a first rougher step of pneumatic froth flotationin the presence of a cationic collecting flotation agent and in thecourse of which finely disseminated air is introduced into a relativelydeep column of the aqueous pulp at a' plurality of superimposed andvertically spaced levels,

removing a silica froth from the top of the pulp column and a rougherdischarging phosphate concentrate in the underflow from the bottom ofthe pulp column, subjecting said rougher phosphate concentrate to asizing treatment to obtain a product of relatively coarse particle sizeand a product of relatively fine particle size, and subjecting each ofsaid sized products to separate cleaner steps of pneumatic frothflotation in the presence of a cationic collecting flotation agentformulated to collect the type of silica predominating in each of saidsized products.

7. The method of separating silica from a mixture of phosphatic materialand silica which comprises subjecting an aqueous pulp of the deslimedand unsized mixture to pneumatic froth flotation in the presence of acationic collecting flotation agent and in the course of which finelydisseminated air is introduced into a relatively deep column of theaqueous pulp at a plurality of superimposed and vertically spaced levelsand water is introduced in a fine state of subdivision into the pulpcolumn at the bottom thereof thereby substantially decreasing thedownward velocity of the pulp flow, removing a silica froth from the topof the pulp column and discharging a phosphate concentrate in theunderflow from the bottom of the pulp column together with theequivalent of most of the water introduced into the bottom of the pulpcolumn, subjecting said phosphate concentrate to a sizing treatment toobtain a product of relatively coarse particle size and a product ofrelatively fine particle size, and subjecting each of said sizedproducts to separate cleaner steps of pneumatic froth flotation in thepresence of a cationic collecting flotation agent formulated to collectthe particular type of silica predominating in each of said sizedproducts, said cleaner steps of pneumatic froth flotation being of thesame character as the aforesaid rougher step of froth flotation.

8. The method of separating silica from a mixture of phosphatic materialand silica which comprises subjecting an aqueous pulp of the deslimedand unsized mixture to pneumatic froth flotation in the presence of acationic collecting flotation agent and in the course of which finelydisseminated air is introduced into a relatively deep column of theaqueous pulp at a plurality of superimposed and vertically spaced levelsand water is introduced in a fine state of subdivision into the pulpcolumn at the bottom thereof thereby substantially decreasing thedownward velocity of the pulp flow, removing a silica froth from the topof the pulp column and discharging a phosphate concentrate in theunderflow from the bottom of the pulp column together with theequivalent of most of the water introduced into the bottom of the pulpcolumn, subjecting said phosphate concentrate to a sizing treatment toobtain a product of relatively coarse particle size predominately minus14 mesh and on 35 mesh and a product of relatively fine particle sizesubstantially all minus 35 mesh and predominately minus mesh, andsubjecting each of said sized products to separate cleaner steps ofpneumatic froth flotation in the presence of a cationic collectingflotation agent formulated to collect the particular type of silicapredominating in each of said sized products, said cleaner steps ofpneumatic froth flotation being of the same character as the aforesaidrougher step of froth flotation.

References Cited in the file of this patent UNITED STATES PATENTS2,156,245 Mead et al Apr. 25, 1939 2,176,107 Smith Oct. 17, 19392,185,224 Ralston Jan. 2, 1940 2,357,419 Mead et a1 Sept. 5, 19442,369,311 Mead et al Feb. 13, 1945 OTHER REFERENCES Handbook of MineralDressing (Taggart), published by John Wiley and Sons, Incorporated (NewYork), 1927, pages 12-14 relied on.

1. THE METHOD OF SEPARATING SILICA FROM AN UNSIZED SILICA-CONTAININGPHOSPHATE PRODUCT WHICH COMPRISES SUBJECTING THE DESLIMED AND UNSIZEDPHOSPHATE PRODUCT TO A FIRST ROUGHER STEP OF PNEUMATIC FROTH FLOTATIONIN THE PRESENCE OF A CATIONIC COLLECTING FLOTATION AGENT AND THE THECOURSE OF WHICH A SILICA FROTH AND A ROUGHER PHOSPHATE